Add note to 'satisfy' manpage about it not satisfying numcopies and how to set it...

Add note to 'satisfy' manpage about it not satisfying numcopies and how to set it up to pass files between remotes that are not present locally.

I got bitten several times in the past by the fact that local preferred
content expressions are not violated (even temporarily) in order to
satisfy numcopies or other remotes' preferred content expressions.
Mostly in the form of the local repo not allowing arbitrary files in
(e.g. because it's set to only want `present` files). This note I add
here explains how to get out of this situation with
`approxlackingcopies=1`.

It might be too specific for this manpage, but I didn't find a better
place to put it.